A
scientific theory is an explanation of an aspect of the
natural world and universe that can be (or
a fortiori, that has been)
repeatedly tested and
corroborated in accordance with the
scientific method, using accepted
protocols of
observation, measurement, and evaluation of results. Where possible, theories are tested under controlled conditions in an
experiment.
[1][2] In circumstances not amenable to experimental testing, theories are evaluated through principles of
abductive reasoning. Established scientific theories have withstood rigorous scrutiny and embody scientific
knowledge.
A scientific theory differs from a
scientific fact or
scientific law in that a theory seeks to explain "why" or "how", whereas a fact is a simple, basic observation and a law is an empirical description of a relationship between facts and/or other laws. For example,
Newton's Law of Gravity is a mathematical equation that can be used to predict the attraction between bodies, but it is not a theory to explain
how gravity works.
[3] Stephen Jay Gould wrote that "...facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts."
[4]
The meaning of the term
scientific theory (often contracted to
theory for brevity) as used in the
disciplines of science is significantly different from the common
vernacular usage of
theory.
[5][note 1] In everyday speech,
theory can imply an explanation that represents an unsubstantiated and speculative
guess,
[5] whereas in a scientific context it most often refers to an explanation that has already been tested and is widely accepted as valid.
[1][2]